CVE-2026-31860: Unhead has a XSS bypass in `useHeadSafe` via attribute name injection and case-sensitive protocol check
Summary
useHeadSafe() can be bypassed to inject arbitrary HTML attributes, including event handlers, into SSR-rendered <head> tags. This is the composable that Nuxt docs recommend for safely handling user-generated content.
Details
XSS via data- attribute name injection
The acceptDataAttrs function (safe.ts, line 16-20) allows any property key starting with data- through to the final HTML. It only checks the prefix, not whether the key contains spaces or other characters that break HTML attribute parsing.
typescript function acceptDataAttrs(value: Record<string, string>) { return Object.fromEntries( Object.entries(value || {}).filter(([key]) => key === 'id' || key.startsWith('data-')), ) }
This result gets merged into every tag's props at line 114:
typescript tag.props = { ...acceptDataAttrs(prev), ...next }
Then propsToString (propsToString.ts, line 26) interpolates property keys directly into the HTML string with no sanitization:
typescript attrs += value === true ? ${key} : ${key}="${encodeAttribute(value)}"
A space in the key breaks out of the attribute name. Everything after the space becomes separate HTML attributes.
PoC
The most practical vector uses a link tag. <link rel="stylesheet"> fires onload once the stylesheet loads, giving reliable script execution:
javascript useHeadSafe({ link: [{ rel: 'stylesheet', href: '/valid-stylesheet.css', 'data-x onload=alert(document.domain) y': 'z' }] })
SSR output:
html <link data-x onload=alert(document.domain) y="z" rel="stylesheet" href="/valid-stylesheet.css">
The browser parses onload=alert(document.domain) as its own attribute. Once the stylesheet loads, the handler fires.
The same injection works on any tag type since acceptDataAttrs is applied to all of them at line 114. Here's the same thing on a meta tag (the injected attributes render, though onclick doesn't fire on non-interactive <meta> elements):
javascript useHeadSafe({ meta: [{ name: 'description', content: 'legitimate content', 'data-x onclick=alert(document.domain) y': 'z' }] })
Realistic scenario
A Nuxt app accepts SEO metadata from a CMS or user profile. The developer uses useHeadSafe() as the docs recommend. An attacker puts a data- key with spaces and an event handler into their input. The payload renders into the HTML on every page load.
Suggested fix
For vulnerability 1, validate that attribute names only contain characters legal in HTML attributes:
typescript const SAFEATTRRE = /^[a-zA-Z][a-zA-Z0-9\-]$/
function acceptDataAttrs(value: Record<string, string>) { return Object.fromEntries( Object.entries(value || {}).filter( ([key]) => (key === 'id' || key.startsWith('data-')) && SAFEATTRRE.test(key) ), ) }
Other sources
Unhead is a document head and template manager. Prior to 2.1.11, useHeadSafe() can be bypassed to inject arbitrary HTML attributes, including event handlers, into SSR-rendered <head> tags. This is the composable that Nuxt docs recommend for safely handling user-generated content. The acceptDataAttrs function (safe.ts, line 16-20) allows any property key starting with data- through to the final HTML. It only checks the prefix, not whether the key contains spaces or other characters that break HTML attribute parsing. This vulnerability is fixed in 2.1.11.
— MITRE
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2026-31860?
CVE-2026-31860 has been classified with a medium severity due to its potential to allow XSS attacks via attribute name injection.
How do I fix CVE-2026-31860?
To fix CVE-2026-31860, upgrade the `unhead` package to version 2.1.11 or later.
What versions of unhead are affected by CVE-2026-31860?
CVE-2026-31860 affects unhead versions up to and including 2.1.10.
What type of attack can be executed due to CVE-2026-31860?
CVE-2026-31860 can be exploited to execute Cross-Site Scripting (XSS) attacks by injecting arbitrary HTML attributes.
Is CVE-2026-31860 applicable to client-side or server-side code?
CVE-2026-31860 is applicable to server-side rendered applications using the `useHeadSafe` composable in unhead.